In 1979 we started a novel application of the spin label ESR method for studying the macroscopic deformation of erythrocytes under a shear stress. During the past one year, our major effort was focussed on proving our idea that the ESR spectral change observed in a cell suspension in a shear flow is a useful measure to assess the red cell deformability. The objective has been accomplished successfully by the empirical approaches. Also, the first attempt was made, with some success, to interpret the nature of the spectral change in the light of available fluid mechanical theories.